The invention relates to an exhaust-gas aftertreatment system for an internal combustion engine, to an internal combustion engine having such an exhaust-gas aftertreatment system, and to use of an air flow nozzle.
In the case of exhaust-gas aftertreatment systems for internal combustion engines, which have a catalytic converter device which is designed for the catalytic conversion of at least one exhaust-gas component with a reactant, the requirement arises that the reactant has to be metered into a flow path of the exhaust gas. For this purpose, a reactant-metering device is typically arranged upstream of the catalytic converter device, as seen along a flow path of the exhaust gas through the exhaust-gas aftertreatment system. For efficient conversion of the exhaust-gas component with the reactant at the catalytic converter device, the reactant has to be intimately mixed with the exhaust gas, preferably evaporated, and optionally converted chemically to form a component which then ultimately reacts at the catalytic converter device. For this purpose, long metering and mixture preparation sections are generally required, especially in the large engine range, which increases the construction space required for the exhaust-gas aftertreatment system. If a turbine, for example of an exhaust-gas turbocharger, is provided in the exhaust-gas aftertreatment system, said turbine can be used as a mixer by the reactant-metering device being arranged upstream of the turbine. This considerably shortens the mixing section. However, it has been shown that the reactant-metering device typically has to operate counter to the exhaust-gas pressure upstream of the catalytic converter device and in particular upstream of the turbine, wherein, in particular in the latter case, a pressure of approximately 5 to 6 bar can prevail in an exhaust-gas line into which the reactant is intended to be metered. If a pressure atomizer is used, the latter requires a reactant admission pressure of at least 10 bar, preferably of more than 10 bar, in order to ensure an atomization quality sufficient for preparation of the reactant in respect of an efficient reaction at the catalytic converter device.
The pressure difference which drops via a pressure atomizer nozzle and is intended for generating a certain atomization quality depends squarely here on the flow of reaction medium through the pressure atomizer nozzle, i.e. on a desired metering quantity. This gives rise to the necessity of a high dynamic range for the admission pressure of the pressure atomizer nozzle. The high pressures in particular required in this respect are achievable only with a comparatively high degree of complexity, which is also associated with high costs. Furthermore, pressure atomizers typically generate a comparatively coarse spray, i.e. in particular comparatively large reactant droplets, and therefore, for the mixture preparation, long residence times in the exhaust-gas aftertreatment system are required in order to completely vaporize the reactant and to mix same intimately with the exhaust gas. Atomizer nozzles are also known which generate a spray with the aid of compressed air. This means a high degree of technical complexity in order to provide compressed air in the form of pressure vessels and/or by means of a compressor.